This type of operation has been described, for example, in U.S. Pat. No. 4,251,295 or U.S. Pat. No. 4,495,060 which describe the H-OIL process.
Hydrotreatment of hydrocarbon feeds, such as sulphur-containing petroleum cuts, is becoming more and more important in refining with the increasing need to reduce the quantity of sulphur in petroleum cuts and to convert heavy fractions into lighter fractions which can be upgraded as a fuel. Both to satisfy the specifications imposed in every country for commercial fuels and for economical reasons, imported crudes which are becoming richer and richer in heavy fractions and in heteroatoms and more and more depleted in hydrogen must be upgraded to the best possible extent. This upgrading implies a relatively large reduction in the average molecular weight of heavy constituents, which can be obtained, for example, by cracking or hydrocracking the pre-refined feeds, i.e., desulphurized and denitrogenated feeds. Van Kessel et al explained this subject in detail in an article published in the review "Oil & Gas Journal", Feb. 16, 1987, pages 55 to 66.
The skilled person is aware that during hydrotreatment of petroleum fractions containing organometallic complexes, the majority of those complexes are destroyed in the presence of hydrogen, hydrogen sulphide, and a hydrotreatment catalyst. The constituent metal of such complexes then precipitates out in the form of a solid sulphide which then becomes fixed on the internal surface of the pores. This is particularly the case for vanadium, nickel, iron, sodium, titanium, silicon, and copper complexes which are naturally present to a greater or lesser extent in crude oils depending on the origin of the crude and which, during distillation, tend to concentrate in the high boiling point fractions and in particular in the residues. This is also the case for liquefied coal products which also comprise metals, in particular iron and titanium. The general term hydrodemetallization (HDM) is used to designate those organometallic complex destruction reactions in hydrocarbons.
The accumulation of solid deposits in the catalyst pores can continue until a portion of the pores controlling access of reactants to a fraction of the interconnected pore network is completely blocked so that that fraction becomes inactive even if the pores of that fraction are only slightly blocked or even intact. That phenomenon can cause premature and very severe catalyst deactivation. It is particularly sensitive in hydrodemetallization reactions carried out in the presence of a supported heterogeneous catalyst. The term "heterogeneous" means not soluble in the hydrocarbon feed. In that case, it has been shown that pores at the grain periphery are blocked more quickly than central pores. Similarly, the pore mouths block up more quickly than their other portions. Pore blocking is accompanied by a gradual reduction in their diameter which increasingly limits molecule diffusion and increases the concentration gradient, thus accentuating the heterogeneity of the deposit from the periphery to the interior of the porous particles to the point that the pores opening to the outside are very rapidly blocked; access to the practically intact internal pores of the particles is thus denied to the reactants and the catalyst is prematurely deactivated.
The phenomenon described above is known as pore mouth plugging. Proof of its existence and an analysis of its causes have been published a number of times in the international scientific literature, for example; "Catalyst deactivation through pore mouth plugging" presented at the 5th International Chemical Engineering Symposium at Houston, Tex., U.S.A., March 1978, or "Effects of feed metals on catalyst ageing in hydroprocessing residuum" in Industrial Engineering Chemistry Process Design and Development, volume 20, pages 262 to 273 published in 1981 by the American Chemical Society, or more recently in "Effect of catalyst pore structure on hydrotreating of heavy oil" presented at the National conference of the American Chemical Society at Las Vegas, U.S.A., Mar. 30, 1982.
A catalyst for hydrotreatment of heavy hydrocarbon cuts containing metals must thus be composed of a catalytic support with a porosity profile which is particularly suitable for the specific diffusional constraints of hydrotreatment, in particular hydrodemetallization.
The catalysts usually used for hydrotreatment processes are composed of a support on which metal oxides such as cobalt, nickel or molybdenum oxides are deposited. The catalyst is then sulphurated to transformed all or part of the metal oxides into metal sulphide phases. The support is generally alumina-based, its role consisting of dispersing the active phase and providing a texture which can capture metal impurities, while avoiding the blocking problems mentioned above.
Catalysts with a particular pore distribution have been described in U.S. Pat. No. 4,395,329. There are two types of prior art alumina-based supports. Firstly, alumina extrudates exist that are prepared from an alumina gel. Hydrotreatment catalysts prepared from such extrudates have a number of disadvantages. Firstly, the process for preparing the alumina gel is particularly polluting, in contrast to that of alumina originating from rapid dehydration of hydrargillite, known as flash alumina. The pores of alumina gel based supports are particularly suitable for hydrodesulphuration and hydrotreatment of light hydrocarbons, and not for other types of hydrotreatment. Further, even though such extrudates are balanced in their hydrodemetallization/hydrodesulphuration ratio, their hydrometallization retention capacity is low, in general at most 30% by weight, so they are rapidly saturated and have to be replaced. Further, considering the high production cost of the alumina gel, the manufacture of such catalysts is very expensive.
Secondly, alumina beads prepared by rapid dehydration of hydrargillite then agglomerating the flash alumina powder obtained are used as a support for catalysts for hydrotreating hydrocarbon feeds containing metals. The cost of preparing these beads is lower, however in order to maintain it at a satisfactory level, beads with a diameter of more than 2 mm have to be prepared. As a result, the metals cannot be introduced right into the core of the beads, and the catalytic phase located there is not used.
Hydrotreatment catalyst prepared from flash alumina extrudates which are smaller than which have a porosity which is suitable for hydrotreatment would not have all of those disadvantages, but there is currently no industrial process for preparing such catalysts.